What is a volcano?
Heat is generated in the earth's core by nuclear processes. The decay of radioactive thorium is believed to be the source of energy. The temperature at the earth's core is estimated at 7,000 to 10,000 degrees Fahrenheit, thus, much of the material below the earth's surface is viscous and molten. Heat is carried up from the core to upper layers nearer the surface by complex convection currents, much like boiling water, but in slow motion.
This heating from below causes mantle and/or crustal rock to soften and melt. Various mechanisms can cause this material to become pressurized and forced upwards to the surface. Voilá, we have a volcano. About magma and lavaMagma refers to molten material that is still in the earth and which has not yet been ejected to the surface. After it is ejected, other terms are used to describe the resulting material. The chemical composition of the magma largely determines the eruptive behavior of a volcano.
Magma which is composed of mantle rock has little dissolved gas. This type of magma erupts as a relatively placid viscous liquid (lava), flows smoothly if sufficiently hot and freezes into a black rock called basalt. The volcanos of Hawaii are of this type.
Volcanoes whose magma source is not mantle rock can exhibit quite different behaviors. Plate tectonics is the gradual movement of crustal plates on the surface of the earth. In certain regions of the earth, two plates move apart creating a rift where volcanic magma can erupt. This process is occuring at the bottom of the Pacific Ocean and the Hawaiian Islands result from this type of volcanism. In other cases, two plates collide. In some cases, the plates crush together raising huge mountain ranges such as the Himalayas of Asia and the Alps of Europe. In other cases, one plate plunges beneath the other in a process called subduction. The plate being subducted gradually bends downward and plunges into the earth where it eventually encounters sufficient heat to soften and melt. Melted rock of this type can sometimes force its way to the surface, forming a volcano.
Magma composed of re-melted surface rock from a subducting plate behaves very differently from the mantle-rock magma mentioned above. Subducting plates often contain ancient seafloor material consisting of thick layers of carbonate minerals. When these materials melt, great quantities of carbon dioxide gas are liberated but are kept dissolved in the magma by the great pressure at depth. (The principle is the same as in a soda pop bottle where pressure keeps the carbon dioxide gas in solution). Gaseous magma also contains quantities of hydrogen, carbon monoxide, sulfur dioxide, and toxic hydrogen sulfide.
When magma of this type finds its way to the surface and the pressure is reduced the carbon dioxide gas comes out of solution, often with explosive results. It is carbon dioxide gas that powers the explosive fireworks of Mt. Stromboli, the rocket engine-like behavior of Mt. Vesuvius, and the vast explosions of calderas like Long Valley in California. Volcanic Ejecta
It is said that the Eskimos of North America have 7,000 words to describe snow and its consistency. The material ejected from a volcano is equally varied, the result depending on temperature, force of the eruption, gas content, mineral composition and other factors resulting in an almost infinite variety. Nevertheless, volcanologists and geologists have classified this material into broad categories, which are described below:
Lava refers to the hot molten or plastic rock which flows or oozes out of a volcanic vent (in contrast to volcanic ash and material which is propelled into air by gas pressure or explosions. When this material flows out and solidifies, the rock formed is categorized into three major types:

Basalt, which is a uniform, usually black material that forms from lava of low acidity.

Rhyolite, which forms from lava with a high acid content.

Obsidian , forms when rhyolitic lava is cooled suddenly on contact with cold rocks, ice (glaciers) or water. Because of the sudden cooling, thus there is no time for crystals to form and the result is volcanic glass. Obsidian was used by American Indians to make edged weapons (knives, arrowheads, etc.). It is still used today to make special, extremely sharp scalpels used for eye surgery.

Rhyolite and basalt result from magma which cooled slowly enough for crystals to form. The slower the cooling process, the larger the crystals become. Granite, with its large crystals forms from acidic magma (like rhyolite) but cooled even more slowly. Gabbro forms from basic (alkaline) magma (like basalt) but with slower cooling giving more time for crystals to form.
Lava also exhibits various shape characteristics once it cools. Volcanologists use the Hawaiian words "pahoehoe" and "aa aa" to describe the two main types of solidifed lava. Pahoehoe refers to lava which has a ropey, plastic, wrinkled appearance. Aa-aa refers to lava which hardens into heaps of shattered rock, reminiscent of broken glass (and equally sharp). Ash
If the eruption is explosive (discussed below) and material is propelled high into the air, this material is called "volcanic ash". Volcanic ash is nothing like the ashes in your fireplace -- it is rock ranging from fine sand grains to larger pieces (up to a couple of inches) called lapilli and even larger pieces called volcanic bombs which can weigh several pounds. Extremely fine volcanic ash can be propelled into the upper atmosphere where it can remain for months or years, circling the earth. Larger ash, like sand, falls back to the ground and can do great damage. A layer of ash a few inches thick is so heavy it can crush the roof of an average home.
The terms ash, lapilli, and bomb refer to the sizes of volcanic material ejected. The consistency of this material is described by additional terms such as pumice which is a frothy material which contains so much trapped gas that it floats on water, scoria which is heavier than water, and spatter which refers to material which is still soft when it hits the ground. Pyroclastic Flows
The term pyroclastic refers to a suspension of hot volcanic material and gases. Under the right circumstances, hot volcanic particles and gases can form a material that behaves like snow in an avalanche (which is a mixture of ice particles and air). This suspension behaves like a liquid. It can accelerate to high velocities (hundreds of miles per hour) and can travel great distances, up and down over hills, following the form of the land and destroying everything in its path. Pyroclastic flows are one of the most dangerous and destructive products of a volcano. Magma with high gas content (such as that found in most volcanos in Guatemala and the U.S. Pacific Northwest) is more likely to produce pyroclastic flows. LaharsLahars are another destructive effect of a volcano. A lahar consists of volcanic particles mixed with water - a type of mudflow in other words. Lahars can form in several ways and may not be associated with an eruption. The mudflow from Volcan de Agua which destroyed the capitol of Antigua Guatemala in 1541 was a lahar. At that time, the crater of the volcano was filled with water. One side of the crater collapsed, releasing the water and creating the mudflow. Lahars can also result when pyroclastic flows encounter bodies of water such as what happened at Mt. Saint Helens in the U.S. Types of Eruptions
Depending on the type of magma feeding the volcano, the host rock, the shape of the volcano's internal plumbing, magma temperature and pressure, and many other factors, eruptions vary widely in character. No two volcanos are exactly alike and each has its own "personality". Nevertheless, in order to be able to discuss eruptive behavior, volcanologists have created several broad classifications, often named for the volcano where the behavior was first observed. Volcanos can also mix styles of eruption and sometimes change style from one moment to the next. Hawaiian eruptions and fissure eruptions result from magma with low gas content. The lava has a relatively low viscosity and erupts relatively quietly. Because of its low-viscosity, it flows out over a wide area and does not form tall cones. If sufficient magma is ejected, over time, tall mountains can form but they have a low, flat dome shape. The entire island of Hawaii is such a volcano and, measured from the seafloor, it is the tallest mountain on earth (over 30,000 feet). This type of magma can be pushed out by high pressure from within the earth and can erupt in tall fountains a thousand feet high or more. When this happens, some of the lava freezes in flight and can form lava bombs, glassy blobs and lapilli (cinders). The pressure which drives the magma in volcanos of this type comes from tectonic and thermal pressure, not the explosive gas pressure which drives the other types of volcanos described below. Plinian eruptions are most exemplified by Mt. Vesuvius in Italy. These volcanos are fed by high-temperature magma which is also high in gas content and during an eruption, produce a relatively steady jet of magma and gas. The internal plumbing of Mt. Vesuvius is a tall, straight tube which permits the rising magma and gas mixture to accelerate to tremendous velocity (supersonic!) by the time it reaches the vent at the top. Plinian volcanos are reminiscent of a huge upside-down rocket engine which can erupt steadily for hours, propelling pumice and ash miles high into the atmosphere that then falls over a wide area. Strombolian eruptions are named after Mt. Stromboli in Italy. These eruptions are characterized by periodic explosions which blow out material of all sizes, ranging from volcanic bombs to ash, up in columns or out in all directions. This type of eruption is reminiscent of aerial fireworks with explosions and chunks of glowing material flying out in all directions, often depicted in paintings of volcanos. Volcán Pacaya near Guatemala City is such a volcano. Vulcanian eruptions are also explosive but result from an interaction between hot magma and groundwater. A typical vulcanian eruption consists of a single explosion (often quite powerful) which ejects ash and steam. The ash is often black in color but the main characteristic is the large quantity of steam released. Volcanic Time Scales
Many of the earth's volcanos are millions of years old and have gone through many cycles of eruption and dormancy. Volcanologists use three terms to describe the short-term behavior of a volcano: Eruption or episode, phase, and pulse.

An eruptive pulse is a short term explosion or burst in activity that lasts from seconds to minutes.

An eruptive phase refers to an increase in activity that can last from hours to days and may consist of many pulses.

An eruptive episode or eruption refers to a period of activity that is composed of one or more phases. An eruption can last from days to years. As a general rule, eruptions which are separated by less than 90 days are considered to be phases of the same episode unless there is a large change in the style of the eruption.

Types of Volcanic ConesShield volcanos are associated with Hawaiian or fissure type volcanos. They result from eruptions of low viscosity lava which spreads out over a wide area and which can gradually build up. The big island of Hawaii is an example of an enormous shield volcano. Shield volcanos sometimes have a caldera at the top resulting from subsidence into the magma chamber below the volcano. Cinder cones (Scoria cones) result from volcanos that propel material into the air which lands and accumulates around the vent. Cinder cones are usually small. Essentially a large pile of rocks, large cinder cones are unstable and prone to landslides. Composite Volcanos (Stratovolcanos) are quite common and often occur in chains. They consist of alternating layers of cinders and lava flows. The lava flows occur both on the surface of the cone and also intrudes into the loose rocks (cinders) forming dikes and sills. The lava flows serve to glue everything together resulting in a much stronger cone and, unlike cinder cones, composite volcanos can grow to great heights. Despite their greater strength and stability, composite cones are still prone to some collapse and landslides. The large and beautiful volcanos of Guatemala such as Volcán de Agua and those surrounding Lake Atitlan are all composite volcanos. Lava Domes are extrusions of viscous magma. These occur most commonly in subduction zones. The magma is so viscous that great columns, hundreds of feet in diameter and hundreds of feet tall can slowly extrude into the air like toothpaste. The magma inside is still very hot and under pressure from the weight of the extruded material above. Such domes can collapse under their own weight. If this happens and the magma inside is still hot, gas is released and a pyroclastic flow can result. Caldera volcanos are the monsters of the volcano world and are in a class by themselves. Fortunately for the human race, these monsters don't erupt very often. Examples of caldera volcanos are Yellowstone National Park and Long Valley, California, both in the U.S. The entire central area of Yellowstone Park is a caldera volcano with a vent 30 miles in diameter!
A classic large caldera volcano essentially consists of a magma chamber located near the surface of the earth, loaded with tens or hundreds of cubic miles of gas rich, highly explosive magma. This magma is kept from exploding by the weight of the skin of rock and soil situated on top of the magma. For an eruption to occur, some rupture of the "skin" takes place, releasing the pressure on the magma below the rupture. This magma explodes and ejects from the vent thereby releasing pressure on the magma just below which in turn explodes and so on in a chain reaction until the magma chamber empties. The magnitude of a large caldera eruption is unimaginable. It can bury hundreds of thousands of square kilometers under a meter thick layer of ash. The last time Yellowstone erupted, the ash fall in parts of Nebraska was a meter deep!
Tiny examples of caldera volcano behavior have been recorded in recent times such as the eruption at Mt. Saint Helens where a small magma chamber a few hundred feet across was uncovered by a landslide and subsequently exploded.
Fortunately, caldera volcanos have eruptive cycles measured in hundreds of thousands of years or millions of years. Both Yellowstone and Long Valley appear to have eruptive cycles of about 600,000 years. Unfortunately, it has been about 600,000 years since their last eruptions.

Lake Atitlán (Lago de Atitlán) is a large endorheic lake (one that does not flow to the sea) in the Guatemalan Highlands. While Atitlan is recognized to be the deepest lake in Central America, its bottom has not been completely sounded. Estimates of its maximum depth range up to 340 meters. The lake is shaped by deep escarpments which surround it and by three volcanos on its southern flank. Lake Atitlan is further characterized by towns and villages of the Maya people.

The lake is volcanic in origin, filling an enormous caldera formed in an eruption 84,000 years ago. It is renowned as one of the most beautiful lakes in the world, and Aldous Huxley famously wrote of it: "Lake Como, it seems to me, touches on the limit of permissibly picturesque, but Atitlán is Como with additional embellishments of several immense volcanoes. It really is too much of a good thing."